In fabricating semiconductor devices, packaging is a process to protect the semiconductor chips from an external environment, to shape a semiconductor chip for an easy application, and to protect the operation functions added in the semiconductor chip and thus enhance the reliability of the semiconductor device.
Recently, as the semiconductor devices are highly integrated and their functions become versatile, the packaging is gradually transferred from a process having a small number of package pins to a process having a large number of package pins, and is also converted from a structure where a package is inserted into a printed circuit board (PCB) to a structure where a package is mounted on a surface of a PCB, i.e., surface mounting device structure. Examples of the surface mounting type packages include a small outline package (SOP), a plastic leaded chip carrier (PLCC), a quad flat package (QFP), a ball grid array (BGA), a chip scale package (CSP) and the like.
A chip carrier related with these semiconductor packages or a base substrate used in a PCB should be stable thermally, electrically and mechanically. As the chip carrier or the base substrate for a PCB, a high price ceramic substrate or a resin substrate having a polyimide resin, a fluorine resin or a silicon resin as a base material has been used.
Since the ceramic substrate or resin substrate is made of insulator, it need not deposit an insulation material after the through hole process. However, in the case of resin substrates, since their material cost is expensive and is poor in the moisture-resistant property and heat-resistant property, it is not good to use the resin substrates as the chip carrier substrate. Also, although the ceramic substrate is comparatively superior in terms of heat-resistant property to the resin substrate, the ceramic substrate is also expensive and has disadvantages of a high processing cost as well as a difficulty in the processing.
To overcome the disadvantages of these ceramic substrates or resin substrates, use of a metallic substrate is proposed. The metallic substrate is advantageous in that it is inexpensive, can be easily processed and has a good thermal reliability. However, the metallic substrate requires a separate insulation treatment, which is unnecessary for the aforementioned ceramic or resin substrates, and also requires to attach a metal core serving as a heat sink or a heat spread on an upper surface or a lower surface of a completed substrate so as to more effectively irradiate heat.
In the meanwhile, the chip carrier or PCB prefers a thin and flat one in accordance with the current design trend toward a lightweight, slim and miniaturized profile. To realize the slimness and flatness, a technique that a cavity is formed in a substrate and a chip or a component is mounted on the formed cavity is employed.
In the case of resin substrates, such a cavity is formed by drilling the resin substrate. However, the drilling method takes much time and high cost in processing the cavity. Also, the cavities as formed may have a large deviation, which allows a mounted component to be leaning to one side and makes it difficult to maintain the flatness. Furthermore, since the resin used as the material of the substrate is poor in thermal and mechanical characteristics, when a component is mounted on the substrate, a serious deformation may be caused due to a stress.